This invention relates generally to lightwave communication networks and, more particularly, to optical cross-connect arrangements for routing optical signals in such networks.
Optical fiber is becoming the transmission medium of choice for many communication networks because of the speed and bandwidth advantages associated with optical transmission. In addition, wavelength division multiplexing (WDM) is being used to meet the increasing demands for higher data rates and more bandwidth in optical transmission applications. In its simplest form, WDM is a technique whereby parallel data streams modulating light at different wavelengths are coupled simultaneously into the same optical fiber. As such, a WDM signal is typically viewed as a composite signal comprising a parallel set of optical channels sharing a single transmission medium, each optical channel using a different frequency (wavelength of light). Although each optical channel actually includes a range of frequencies (wavelengths), those skilled in the art typically refer to an optical channel in terms of its center wavelength. Moreover, the terms xe2x80x9coptical signalxe2x80x9d, xe2x80x9coptical channelxe2x80x9d, xe2x80x9cwavelength channelxe2x80x9d, and wavelength are sometimes used interchangeably in the WDM context to refer to a constituent optical signal within the composite WDM signal. Similarly, in a non-WDM context, the term xe2x80x9coptical signalxe2x80x9d is typically used to refer to a single wavelength of light (e.g., single optical channel, single wavelength channel, etc.).
In communication networks, it is sometimes desirable to selectively route individual optical signals or WDM signals to different destinations. As is well known, the component typically used for selectively routing signals through interconnected nodes in a communication network is a high capacity optical switch matrix or cross-connect switch. Because of the aforementioned speed and bandwidth advantages associated with transmitting information in optical form, all-optical network elements are emerging as the preferred solutions for optical networking. Moreover, all-optical network elements are needed to provide the flexibility for managing bandwidth at the optical layer (e.g., on a wavelength by wavelength basis). Accordingly, all-optical cross-connects are being contemplated for use in these networks. However, despite the amount of attention that is being given to the specific candidate technologies for implementing the optical cross-connect fabrics, much less attention has been given to the management of the optical cross-connect fabric.
For example, the traditional way of managing connections in a cross-connect fabric is based on using a cross-connect controller in conjunction with a cross-connect map. As is well-known, a cross-connect map specifies input-to-output routing of optical signals passing through the cross-connect fabric and is typically based on a target network configuration. By way of example, a controller responds to routing requests (e.g., from an operations support system, adjacent cross-connects, other network elements, etc.) and establishes routing paths between inputs and outputs of the cross-connect fabric according to mappings set up in the cross-connect map.
Most methods for verifying cross-connections of optical signals have been limited to the use of the information provided in the cross-connect map. As used herein, verification is meant to refer to the act of verifying that the connections being made from the cross-connect inputs to outputs are correct. For example, in response to a query from the operations support system as to whether a cross-connection has been properly set up in the fabric, the controller typically will only query the cross-connect map, i.e., interrogate the connections specified in the cross-connect map, instead of checking the actual inputs and outputs of the fabric. In these schemes, the controller assumes that the image (e.g., the connections specified in the cross-connect map and displayed at the controller) represents the actual connection paths in the fabric. However, in effect, only the cross-connect map itself is being verified without any independent verification of the actual paths set up in the fabric.
In existing optical transmission applications, other techniques are presently used to verify routing of individual signals. For example, systems based on the well-known Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) standard rely on information carried within the individual signals. In particular, selected bytes (e.g., J0/J1 bytes) in SONET overhead are reserved for routing information, channel identification, and the like. However, this information is only available in electronic form, that is, in bits/bytes that are extracted from digital overhead after the optical signals have been converted to electrical signals (i.e., after optical to electrical (O/E) conversion). Among other disadvantages, this approach can become quite costly because of the need for high speed circuitry for optical to electrical conversion and electronic signal processing for each signal at each input/output port. In addition to cost, implementation can become quite complex, especially for high capacity optical cross-connects having many inputs/outputs. Furthermore, operating in the electrical domain to access the payload bit stream for verifying signal routing defeats the whole purpose of all-optical network elements, e.g., optical cross-connects, which are designed to capitalize on the benefits of all-optical transmission and signal routing. Additionally, some signals used in other applications, e.g., Internet Protocol-based, are not inherently in SONET/SDH format and, consequently, do not even have the J0/J1 byte type of overhead for verifying routing information even in the electrical domain.
Verification that an optical signal has been properly routed from an input to an output of an optical cross-connect is achieved according to the principles of the invention by independently tagging an optical signal (e.g., wavelength) with identification information at a cross-connect input, retrieving the identification information from the tagged optical signal at a cross-connect output, and determining from the retrieved identification information whether the optical signal was routed according to a predetermined route.
In one illustrative embodiment, a connection verification message is created for each cross-connection based on predetermined routes defined in a cross-connect map. The connection verification message can include a message identification, incoming and outgoing wavelength information, incoming and outgoing port information, a time stamp, user verification data, and so on. At each input to the cross-connect, the respective connection verification message is tagged onto its respective optical signal or signals. By way of example, tagging can be accomplished by directly modulating the envelope of the optical signal or by modulating the optical signal with a low frequency subcarrier that has been modulated with the connection verification message. At each cross-connect output, the message (tag) is retrieved or removed and information in the retrieved message for each optical signal is then compared with the cross-connection specified in the cross-connect map to determine whether the optical signals were routed correctly. If not, then remedial actions can be initiated, e.g., notify operations support system or adjacent network element, tear-down connection, etc.
In contrast to prior connection verification arrangements, optical-to-electrical or electrical-to-optical signal conversions for accessing the payload (e.g., high speed data) are avoided and actual connections between inputs and outputs are independently verified. Moreover, by applying the tag to the optical signal, routing verification according to the principles of the invention can be used in cross-connect applications employing one optical signal (i.e., wavelength) at each input and output or applications employing a WDM signal having a plurality of optical channels of different wavelengths at each input and output.